U.S. patent number 5,236,740 [Application Number 07/693,234] was granted by the patent office on 1993-08-17 for methods for coating adherent diamond films on cemented tungsten carbide substrates.
This patent grant is currently assigned to National Center for Manufacturing Sciences. Invention is credited to Robert H. Cummings, Michael G. Peters.
United States Patent |
5,236,740 |
Peters , et al. |
August 17, 1993 |
Methods for coating adherent diamond films on cemented tungsten
carbide substrates
Abstract
A cemented tungsten carbide substrate is prepared for coating
with a layer of diamond film by subjecting the substrate surface to
be coated to a process which first removes a small amount of the
tungsten carbide at the surface of the substrate while leaving the
cobalt binder substantially intact. Murakami's reagent is presently
preferred. The substrate is then subjected to a process which
removes any residue remaining on the surface as a result of the
performance of the process which removes the tungsten carbide. A
solution of sulfuric acid and hydrogen peroxide is presently
preferred. A diamond coated cemented tungsten carbide tool is
formed using an unpolished substrate, which may be prepared by
etching as described above or by etching in nitric acid prior to
diamond film deposition. Deposition of a substantially continuous
diamond film may be accomplished by reactive vapor deposition,
thermally assisted (hot filament) CVD, plasma-enhanced CVD, or
other techniques.
Inventors: |
Peters; Michael G. (Santa
Clara, CA), Cummings; Robert H. (Santa Clara, CA) |
Assignee: |
National Center for Manufacturing
Sciences (Ann Arbor, MI)
|
Family
ID: |
24783865 |
Appl.
No.: |
07/693,234 |
Filed: |
April 26, 1991 |
Current U.S.
Class: |
427/249.13;
427/307; 427/309; 427/902; 216/108; 216/100 |
Current CPC
Class: |
C23F
1/02 (20130101); C23F 1/28 (20130101); C23C
16/0227 (20130101); C23C 16/27 (20130101); Y10T
428/31678 (20150401); Y10S 427/103 (20130101); Y10T
428/30 (20150115); Y10T 428/26 (20150115) |
Current International
Class: |
C23C
16/02 (20060101); C23C 16/27 (20060101); C23C
16/26 (20060101); C23C 016/02 (); C23C 016/26 ();
C23F 001/26 () |
Field of
Search: |
;427/255,249,248.1,307,309 ;156/651,656 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
63-053269 |
|
Mar 1988 |
|
JP |
|
63-100182 |
|
May 1988 |
|
JP |
|
1-201475 |
|
Aug 1989 |
|
JP |
|
Primary Examiner: Beck; Shrive
Assistant Examiner: Chen; Bret
Attorney, Agent or Firm: D'Alessandro; Kenneth
Claims
What is claimed is:
1. A process for coating an unpolished cemented tungsten carbide
substrate with a diamond film, comprising the steps of:
performing a first etching step comprising etching an unpolished
cemented tungsten carbide substrate in Murakami's reagent;
performing a second etching step comprising etching said unpolished
substrate in a solution of sulfuric acid and hydrogen peroxide;
depositing a substantially continuous diamond film on a selected
portion of said surface of said unpolished substrate.
2. A process for coating an unpolished cemented tungsten carbide
substrate with a diamond film, comprising the steps of:
performing a first etching step comprising etching an unpolished
cemented tungsten carbide substrate in Murakami's reagent;
performing a second etching step comprising etching said unpolished
substrate in a solution of sulfuric acid and hydrogen peroxide;
depositing a substantially continuous CVD polycrystalline diamond
film on a selected portion of said surface of said unpolished
substrate.
3. The process of claim 1 wherein said first etching step comprises
an etch in a solution comprising 10 grams of potassium ferricyanide
and 10 grams of potassium hydroxide per 100 ml of water for a
period greater than about two minutes and wherein said second
etching step comprises an etch in a solution of about 30 volume
percent sulfuric acid and about 70 volume percent hydrogen peroxide
for a period greater than about 5 seconds.
4. The process of claim 2 wherein said first etching step comprises
an etch in a solution comprising 10 grams of potassium ferricyanide
and 10 grams of potassium hydroxide per 100 ml of water for a
period greater than about two minutes and wherein said second
etching step comprises an etch in a solution of about 30 volume
percent sulfuric acid and about 70 volume percent hydrogen peroxide
for a period greater than about 5 seconds.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to hard coatings for tools. More
particularly, the present invention relates to diamond coated
cemented tungsten carbide articles and methods for making diamond
coated cemented tungsten carbide articles.
2. The Prior Art
Polycrystalline diamond (PCD) cutting tools, comprising a piece of
polycrystalline diamond fastened to the tip of a tool insert are
known in the art. These tools are expensive to manufacture and do
not readily lend themselves to indexing for increased tool life. In
addition, PCD tooling having complex shapes, i.e., taps, drill
bits, cannot be formed using any known techniques. PCD tools are
typically run at cutting speeds of around 2,500 SFM when cutting
materials such as 390 aluminum.
Numerous attempts have been made to provide diamond coated tools
which have performance approaching that of PCD tools because they
would be less costly to manufacture and use, and because diamond
coated tools having more complex shapes than are possible with PCD
tools are theoretically manufacturable employing substrates such as
cemented tungsten carbide.
A significant challenge to the developers of diamond-coated tooling
is to optimize adhesion between the diamond film and the substrate
to which it is applied, while retaining sufficient surface
toughness in the finished product. Substrates like Si.sub.3
N.sub.4, and SiAlON can only be formed into a few geometries,
limiting their commercial potential. Sintered tungsten carbide (WC)
substrates without cobalt or other binders have been studied but
can be too brittle to perform satisfactorily as tooling in
machining applications.
Cemented tungsten carbide substrates incorporating a cobalt binder
in concentrations between about 4% and 6% (WC/Co) have the
requisite toughness and thus show the greatest long-term commercial
promise for tooling applications. A cemented tungsten carbide
substrate with up to 6% cobalt would provide adequate surface
toughness for most machining tasks. Cemented tungsten carbide can
be formed into a variety of geometries, making it a potential
material for drilling operations, die manufacturing, and other
applications of value to the automobile and other industries. It is
therefore desireable to provide a way to coat cemented tungsten
carbide substrates with a layer of diamond film having adequate
adhesion to the substrate for use as a machine tool.
It has been reported in the literature that the use of a cobalt
binder in cemented carbides inhibits adhesion of the diamond film
to the substrate. R. Haubner and B. Lux, Influence of the Cobalt
Content in Hot-Pressed Cemented Carbides on the Deposition of
Low-Pressure Diamond Layers, Journal De Physique, Colloque C5,
supplement au no. 5, pp. C5-169-156, Toma 50, May 1989. Indeed,
conventional wisdom indicates that successful use of cemented
tungsten carbide substrates may only be achieved by utilizing
substrates containing no cobalt, as taught in U.S. Pat. No.
4,990,403; no more than 4% Co binder, as taught in U.S. Pat. No.
4,731,296, or by deliberately depleting the cobalt concentration at
the surface of the substrate. It is known to deplete the cobalt
concentration at the surface of the substrate by selective etching
or other methods, M. Yagi, Cutting Performance of Diamond Deposited
Tool For Al 18 mass % Si Alloy, Abst. of 1st Int. Conf. on the New
Diamond Sci. & Technol., pp. 158-159, Japan New Diamond Forum,
1988., but this decreases the surface toughness of the substrate
and can cause chipping of the substrate and applied diamond film.
Increased adhesion of diamond to the substrate may be achieved by
decarburizing the substrate prior to deposition, as taught in
European Patent Application Publication No. 0 384 011, but use of
this procedure does not optimize substrate toughness and does not
lend itself well to manufacturing environments where repeatability
and consistency are important issues.
The prior art teaches polishing or scratching the surface of a
cemented tungsten carbide substrate prior to attempting diamond
deposition due to the enhancement to the nucleation process caused
by scratching and polishing. Haubner and Lux; Yagi; M. Murakawa et
al., Chemical Vapour Deposition of A Diamond Coating Onto A
Tungsten carbide Tool Using Ethanol, Surface Coatings Technology,
Vol. 36, pp. 303-310, 1988; Kuo et al., Adhesion and Tribological
Properties of Diamond Films on various substrates, J. Mat. Res.,
Vol. 5, No. 11, November 1990, pp. 2515-2523. These articles either
teach use of polished substrates or indicate poor results obtained
by utilizing substrates whose surfaces have not been prepared by
polishing or scratching.
A promising solution to the adhesion problem has been to employ an
interlayer between the diamond and a WC/Co substrate. This
encapsulates the Co, optimizing adhesion while allowing the
substrate to retain its toughness. It may also be possible to
choose an effective interlayer material that bonds strongly to
diamond, further increasing adhesion. U.S. Pat. No. 4,707,384
discloses use of a titanium carbide interlayer. U.S. Pat. Nos.
4,998,421 and 4,992,082 disclose utilization of a plurality of
layers of separated diamond or diamond like particles interposed
with layers of a planarized bonding material.
It would, however, be advantageous to develop a direct diamond
coated cemented tungsten carbide article having superior adhesion
for machining purposes. A process for producing such a
diamond-coated cemented tungsten carbide article would also be
desireable.
BRIEF DESCRIPTION OF THE INVENTION
According to a first aspect of the present invention, a cemented
tungsten carbide substrate is prepared for coating with a layer of
diamond film by subjecting the substrate surface to be coated to a
process which first removes a small amount of the tungsten carbide
at the surface of the substrate while leaving the cobalt binder
substantially intact. The substrate is then subjected to a process
which removes any residue remaining on the surface as a result of
the performance of the process which removes the tungsten carbide.
As presently preferred, the first step of the process is carried
out by selectively etching tungsten carbide at the surface of the
substrate with Murakami's reagent and the second step of the
process is carried out by an etch in a solution of nitric acid in
water or a solution of sulfuric acid and hydrogen peroxide.
According to a second aspect of the present invention, a diamond
coated cemented tungsten carbide tool is formed using an unpolished
substrate. Prior to diamond film deposition, the substrate may be
treated according to the first aspect of the invention or may be
etched in nitric acid. Diamond film deposition may be accomplished
by reactive vapor deposition, thermally assisted (hot filament)
CVD, or plasma-enhanced CVD. In a presently preferred embodiment, a
plasma-enhanced microwave CVD deposition process is employed at a
substrate temperature greater than 900.degree. C.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Those of ordinary skill in the art will realize that the following
description of the present invention is illustrative only and not
in any way limiting. Other embodiments of the invention will
readily suggest themselves to such skilled persons.
According to a first aspect of the present invention, a method is
described for preparing a cemented tungsten carbide substrate for
coating with a layer of diamond film. The substrate is first
treated by removing a small amount of the tungsten carbide at the
surface of the substrate while leaving the cobalt binder
substantially intact. In a presently preferred embodiment of the
invention, the substrate is first degreased using a standard
solvent cleaning step employing, for example, TCA, TCE and/or
acetone. The substrate is then rinsed with DI water.
The substrate is then etched to selectively remove tungsten carbide
at the surface of the substrate. In a presently preferred
embodiment, the etch is performed using Murakami's reagent (10
grams of potassium ferricyanide, 10 grams of potassium hydroxide,
and 100 ml of water) for a period greater than two minutes,
preferably from about 3-80 minutes. The substrate is then rinsed
with DI water.
A second etching step in the process is performed to remove any
residue from the Murakami's reagent and some of the cobalt binder.
As presently preferred, this step comprises an etch for about 10
seconds in a solution of sulfuric acid and hydrogen peroxide.
Etching times as short as 5 seconds in 1-5 volume percent sulfuric
acid in solution with hydrogen peroxide and as long as 30 seconds
in 50 volume percent sulfuric acid hydrogen peroxide solutions have
been used. The substrate is then again rinsed with DI water.
The substrate is then coated with a diamond film. It has been
discovered that superior results may be obtained using cemented
silicon carbide substrates by leaving the substrate unpolished.
Polishing, scratching and otherwise seeding the substrate are not
performed prior to diamond deposition according to the present
invention. As used herein, "polishing" shall mean polishing,
scratching, or seeding the surface of a substrate. An "unpolished"
substrate shall refer to a substrate which has not been polished,
scratched, or otherwise seeded.
As presently preferred, diamond deposition is performed by
microwave plasma-enhanced CVD techniques in an atmosphere of
between about 1 to 8% methane at a substrate temperature of between
about 900.degree. to 1,000.degree. C., preferably about 950.degree.
C., for a period sufficient to form a substantially continuous
diamond film layer. It is presently preferred to form a diamond
film having a thickness of at least 10 microns, preferably between
about 10 to 30 microns, although other thicknesses may be used.
Pressures in the reactor vessel should be tailored to maintain a
sufficient plasma localized around the substrate without either
extinguishing the plasma or allowing it to jump to the vessel wall.
At a microwave power of 1,500 watts, a pressure of about 80 torr
has been found to be satisfactory, although those of ordinary skill
in the art will be able to determine appropriate pressure and power
combinations.
While it is presently preferred to deposit the diamond film
according to the present invention by using microwave
plasma-enhanced CVD techniques, it is believed that other known
techniques for depositing diamond films, including but not limited
to plasma-enhanced CVD, hot filament CVD and reactive vapor
deposition techniques may be used to perform the processes and
produce the articles of the present invention.
The following examples are instructive to illustrate the principles
of the present invention. Those of ordinary skill in the art will
recognize that the scope of the invention is not limited to the
particular illustrations given in the examples.
EXAMPLE 1
One unpolished 6% cobalt cemented tungsten carbide substrate
comprising a TPG-432A grade H-21 (C-2 class) tool insert from
Teledyne Firth Sterling of La Vergne Tennessee was first washed
with DI water and then degreased using first TCA and then acetone.
The substrate was then etched for six minutes with Murakami's
reagent. The substrate was then again washed with DI water. The
substrate was then etched for approximately 5 seconds in a solution
of 30 volume percent sulfuric acid and 70 volume percent hydrogen
peroxide. The substrate was then again rinsed with DI water.
The substrate was then coated with approximately 15 microns of
diamond film using microwave plasma-enhanced CVD at a power of
1,500 watts in an atmosphere of 1.8% methane for sixteen hours at a
substrate temperature of about 950.degree. C. Compared to untreated
substrates, the adhesion of the diamond film to the tungsten
carbide substrate was superior to coated substrates which did not
undergo the treatment.
EXAMPLE 2
Three unpolished 6% cobalt cemented tungsten carbide substrates
comprising a TPG-432A grade H-21 (C-2 class) tool insert were first
washed in DI water and then degreased as in Example 1. The first
sample was then etched for 30 seconds in 5 volume percent nitric
acid and then rinsed with DI water. The second sample (B) was then
etched for six minutes in Murakami's reagent. It was then rinsed in
DI water. The substrate was then etched for 5 seconds in 50 volume
% nitric acid. It was then again rinsed with DI water. The third
sample (C) was bead blasted and then etched for 30 seconds in 5
volume percent nitric acid and then rinsed with DI water.
The three substrate were then coated with approximately 20 microns
of diamond film using microwave plasma-enhanced CVD at a power of
1,500 watts in an atmosphere of 3% methane for sixteen hours at a
substrate temperature of about 950.degree. C. Upon cooling, the
diamond films delaminated from samples (A) and (C). The diamond
film was adherent to the substrate of sample (B). Sample (B) later
successfully turned a sample of 390 aluminum (three passes on a
sample measuring 7.25 inches in length and approximately 3.5 inches
in diameter at 4,500 SFM, at 0.025 inch depth of cut and 0.005
inch/revolution) without delaminating.
EXAMPLE 3
Three unpolished 6% cobalt cemented tungsten carbide substrates
comprising TPG-432A grade H-21 (C-2 class) tool inserts were first
rinsed in DI water and then degreased as in Example 1. The first
sample (A) received no further substrate preparation prior to
diamond deposition. The second sample (B) was then etched for four
minutes in concentrated nitric acid and then rinsed in nitric acid.
The third sample (C) was polished with 0.1 micron diamond powder.
It was then rinsed in DI water and rubbed with a glove to remove
any loose debris.
The three substrates were then coated with approximately 15 microns
of diamond film using microwave plasma-enhanced CVD at a power of
1,500 watts in an atmosphere of 1.8% methane for sixteen hours at a
substrate temperature of about 950.degree. C. Upon cooling, the
diamond films were adherent to substrates (A) and (B), while the
diamond film delaminated from substrate (C). Both samples (A) and
(B) and later turned a sample liner of 390 aluminum (one pass at
2,000 SFM, 0.025 inch depth of cut, 0.005 inch/revolution, 10 inch
length and approximately 3.5 inches in diameter) without film
delamination. Sample (B) subsequently turned an additional nine
passes on 390 aluminum at 2,500 SFM, 0.025 inch depth of cut, 0.005
inch/revolution, 7.5 inches in length and approximately 3.5 inches
in diameter without failure.
EXAMPLE 4
Three unpolished 6% cobalt cemented tungsten carbide substrates
comprising TPG-432A grade H-21 (C-2 class) tool inserts were first
rinsed in DI water and then degreased as in Example 1. They were
then etched for 30 seconds in 5 volume percent nitric acid. The
first sample (A) received no further substrate preparation prior to
diamond deposition. The second sample (B) was then polished with
0.1 micron diamond powder. It was then rinsed in DI water and
rubbed with a glove to remove any loose debris. The third sample
(C) was polished with 0.1 micron diamond powder. It was then rinsed
in DI water and agitated in an ultrasonic cleaner to remove any
loose debris. No glove touched the region of the substrate on which
diamond was to be deposited.
The three substrates were then coated with approximately 15 microns
of diamond film using microwave plasma-enhanced CVD at a power of
1,500 watts in an atmosphere of 2.4% methane for thirteen hours at
a substrate temperature of about 950.degree. C. Upon cooling, the
diamond film was adherent to substrate (A), while the diamond films
delaminated from substrates (B) and (C).
EXAMPLE 5
Two unpolished 6% cobalt cemented tungsten carbide substrates
comprising TPG-432A grade H-21 (C-2 class) tool inserts were first
rinsed in DI water and then degreased as in Example 1.
The first substrate was then coated with approximately 15 microns
of diamond film using microwave plasma-enhanced CVD at a power of
1,500 watts in an atmosphere of 1.8% methane for sixteen hours at a
substrate temperature of about 950.degree. C. Upon cooling, the
diamond film was adherent to the substrate. The insert subsequently
turned 390 aluminum without film delamination.
The second substrate was coated with approximately 8 microns of
diamond film using microwave plasma-enhanced CVD at a power of
1,200 watts in an atmosphere of 1.8% methane for sixteen hours at a
substrate temperature of about 850.degree. C. Upon cooling, the
diamond film delaminated from the second substrate.
EXAMPLE 6
An unpolished 6% cobalt cemented tungsten carbide substrate
comprising TPG-432A grade H-21 (C-2 class) tool inserts was first
rinsed in DI water and then degreased as in Example 1. The
substrate was then etched for ten minutes with Murakami's reagent
and then rinsed with DI water. The substrate was then etched for 30
seconds in a solution of 30 volume percent sulfuric acid and 70
volume percent hydrogen peroxide. The substrate was then rinsed
with DI water.
The substrate was then coated with approximately 25 microns of
diamond film using microwave plasma-enhanced CVD at a power of
1,100 watts in an atmosphere of 1.8% methane for sixteen hours at a
substrate temperature of about 950.degree. C. The film was adherent
upon cooling. The insert subsequently turned uninterrupted cuts on
a piece of 390 aluminum 7.25 inches in length, 3.5 inches in
diameter having two half inch wide lengthwise slots machined into
it (three passes at 660 SFM and a single pass at 1,000 SFM, with a
0.020 inch depth of cut, 0.004 inch/revolution) without film
delamination.
Generally, use of the substrate preparation procedure of the
present invention allows deposition of thicker diamond films on
cemented tungsten carbide substrates without delamination. Films of
thicknesses up to 50 microns have been successfully deposited using
the methods of the present invention. Without this treatment, it is
difficult to grow films thicker than about 20 microns without
delamination problems.
As can be seen from the examples set forth herein, it has been
discovered that polishing of substrates prior to deposition
(Substrate (C) in Example 3 and both substrates (B) and (C) in
Example 4) results in weakly adhered films which easily delaminate.
In addition, it has been discovered that etching of the substrate
in Murakami's reagent followed by etching in a solution of 30
volume percent sulfuric acid and 70 volume percent hydrogen
peroxide prior to deposition result in superior films. It is
believed that this treatment may be characterized as a removal of a
small amount of the tungsten carbide at the surface of the
substrate while leaving the cobalt binder substantially intact,
followed by removal of any residue from the Murakami's reagent and
some of the cobalt binder. Finally, even without the
above-described treatment, it has been discovered that etching of
an unpolished substrate in nitric acid prior to deposition results
in improved adhesion of the diamond film.
The performance results reported in the prior art for diamond
coated cemented tungsten carbide tools include results obtained
employing cutting speeds of from about 660 to about 1,020 SFM.
These speeds fall substantially short of the typical cutting speeds
encountered with PCD tools. As demonstrated in the examples
included herein, the cutting speeds of the diamond coated cemented
tungsten carbide articles made as disclosed herein are
substantially higher than those reported in the prior art, and are
comparable to the typical PCD tool cutting speeds. Example 6
demonstrates successful interrupted cutting using the tools of the
present invention, a feat not reported in the literature for
diamond coated tools. The slower cutting speeds used in the example
are similar to the slower cutting speeds used for interrupted
cutting with PCD tools.
While embodiments and applications of this invention have been
shown and described, it would be apparent to those skilled in the
art that many more modifications than mentioned above are possible
without departing from the inventive concepts herein. The
invention, therefore, is not to be restricted except in the spirit
of the appended claims.
* * * * *